The present invention relates to coating compositions containing polysiloxane resin-forming precondensates and to articles coated with such compositions.
Various polysiloxanes have been used as major or minor constituents of coating compositions, normally blended with other materials which affect the properties of the coatings. See U.S. Pat. Nos. 3,655,565--McDonald (1972) and 3,062,764--Osdal (1962) for disclosures of the technology of polysiloxane-containing composites. Osdal uses small amounts of polysiloxane resins to prevent mud cracks in fluorocarbon coatings. However, it has been found that polysiloxane-based resin coatings have a tendency to stress craze or form cracks when they are applied in relatively thick coatings for increased durability, such as with thicknesses above 20 microns (.mu.m).
Polysiloxane homopolymer resins and more costly copolymer resins such as those reacted with epoxies are sometimes used as nondurable food release coatings in combination with substantial proportions of silicone oils. The oils are fluids which facilitate application of the coatings and improve release of burned-on food residues. Such coatings are often formulated for relatively low temperature cure. Applications for such coatings include bakery pans which can be reacted frequently or even before each use. See U.S. Pat. Nos. 3,419,514--Hadlock (1968), 3,308,080--Haenni (1967) and 3,801,552--Vasta (1974). The polysiloxane fluid improves release performance of such coatings, but coatings relying on the fluids have limited durability and may not be used many times without recoating. Typical applications of durable coatings are household cookware for top-of-the-range cooking or oven baking which are provided with a nonstick coating that lasts for the life of the item of cookware.
Special siloxanes, homopolymers and copolymers and mixtures of organopolysiloxanes with such organic materials as polyethers are sometimes used for nonstick coatings for bread pans or for consolidated materials resistant to thermal and environmental stresses such as molding compounds. See U.S. Pat. Nos. 2,672,104 and 2,672,105--both Clark (1954). U.S. Pat. No. 3,423,479--Hendricks (1969) discloses polyether coating compositions containing up to 30% polysiloxanes, and U.S. Pat. No. 3,536,657--Noshay et al. (1970) discloses coating compositions containing polyethers and polyether-polysiloxane copolymers. In both patents, the polyethers include particulate polysulfones, and the coatings are generally thermoplastic.
Carbon black and powdered graphite have been added to thin polysiloxane coatings to improve the heat transfer characteristics, U.S. Pat. No. 3,078,006--Price et al. (1963).
Some attempts to make a durable coating for cookware are described in Defensive Publication No. T937,005--Vasta published Aug. 5, 1975 by the U.S. Patent and Trademark Office. A solution of polysulfone resin is mixed with up to 25% by weight of the total of a silicone oil or a silicone resin to produce coatings typically 50 .mu.m thick, about 2 mils, but up to 250 .mu.m (10 mils).
It would be desirable to have a more durable polysiloxane-resin based coating which could be applied in thick layers without being subject to craze cracking and without the necessity of using multiple thin layers.